A few days ago, Sergio, one of my regular commenters, posted a question about programmatically adding new columns to a table. My reply to his comment quickly turned into something that I thought should be promoted to a blog post so here it is.
This question required some investigation because it led me to the discovery of a bug related to adding/removing instances of a table column in a form viewed in a version of Acrobat prior to 8.0. More on that later in this post.
The short answer to Sergio’s question is that yes, in fact, you can modify the set of columns in a table programmatically at runtime. You can do this by either using the presence attribute — although this isn’t recommended because it can lead to data merging problems — or you can use Instance Managers to do it, which is the recommended method to use.
Here’s a sample form that contains a table with a “repeatable column”. Using the add and remove buttons that are provided, you can add and remove instances of the 3rd column.
Minimum Requirements: Designer 7.1, Acrobat 7.0.5.
To better understand what’s going on here, I’ll start by explaining how tables are represented in XFA. In reality, there are no <table>, <tr> or <td> elements in XFA. What happens is that <subform layout=”table|row”> elements get used. When a subform represents a table, it’s layout attribute is set to table and when it represents a row, it’s set to row.
If you read about using a subform’s Instance Manager in order to add/remove instances, you should know that a subform only becomes dynamic or repeatable once it’s placed in a flowed container and its set as repeatable using the Object palette’s Binding tab. By default, a <subform layout=”table”> element is flowed from top to bottom. Since the table subform contains row subforms which, in turn contain the cells, that means that every row in a table could easily be set as repeatable and then get an Instance Manager that you could use to add/remove instances of a particular row.
Just like the table subform, a <subform layout=”row”> is flowed. Its flow direction, however, isn’t top to bottom, it’s right to left. This means that its contents — the cells — are flowed.
Cells and Columns
Table cells are little different. Essentially, since they reside in a row subform which is flowed from right to left, there’s no specific definition for what is a table cell. It’s simply any type of object (text, numeric field, subform, button, etc.) that’s placed within a row subform. The number of objects in the row determines the number of columns there are. Therefore, to have a 3×3 table, you would need 3 row subforms each containing 3 objects.
There’s no actual definition for a table column either. Table columns are inferred by the cells (put simply, without going into details about column spanning). This means that a column consists of objects in separate row subforms which are above and below each other (e.g. the first object on every row make-up the first column in a table).
Given the explanations in my little “table primer” above, we now know that table cells are objects which are flowed from right to left within row subforms. Since row subforms are flowed by nature, it means that if a cell were a subform itself, it could be made repeatable and it would then get its own Instance Manager for free! And once we have Instance Managers, we can start adding and removing instances of those cells.
So the trick here lies in converting every cell which constitutes a column into a subform which then contains the type of object you would normally use to display information in that cell (e.g. a text field, a check box, etc.).
Now if you’ve never noticed the Type property on the Field tab in the Object palette before, you’ll want to check it out because it’s about to come in real handy for setting-up a dynamic table column which consists of cells which are all subforms. This property is used to change the type of the selected object(s). For instance, if you put a text field on a form and decide that it should have been a numeric field, you can change it’s type from text field to numeric field simply by using this property. Typically, you cannot change anything into a subform but when you select a table cell which is a text object (the default cell object type when you insert a new table into a form using the Insert Table menu command or the Table object in the Library’s Standard tab), you can, in fact, change it into a subform. So just select each cell in the column and change its type to a subform using this property. The result will be a cell which is a subform that contains a text object. Then you can change the text object into some other field type which better represents the data which will go into the cell.
Making Cell Subforms Repeatable
Unfortunately, this is one case where you’ll have to use the XML Source tab because the repeatable property isn’t available for cell subforms on the Object palette’s Binding tab. Since it’s a valid XFA setting, you can set this yourself using the XML Source.
Switch to the XML Source window by selecting “XML Source” from the top-level View menu. Then, insert the following XML inside each subform which defines a cell:
This means that the cell subform goes from looking like this:
<subform layout="tb" name="Col3SF"><field name="Cell3" w="29.9999mm" h="10mm">
to looking like this:
<subform layout="tb" name="Col3SF"><occur max="-1"/><field name="Cell3" w="29.9999mm" h="10mm">
and signifies that the cell will have one initial and minimum instance and can have as many additional instances as you need (no maximum).
Last but not least, the script!
Then, to remove an instance of the column, just do the reverse: Use the Instance Manager’s removeInstance method to remove an instance of each cell subform.
Of course, everything was going great until now… Unfortunately, I discovered a little snag in playing with table column instances while making this sample. Fortunately, the bug has already been fixed in Acrobat 8.
The first manifests itself when the second new instance of a column is added (by adding an instance of a column, I mean adding an instance of each cell subform in each row which collectively constitute a table column — as in my sample script above earlier). As of the second new instance, the cell in the header row will stop appearing in all subsequent instances.
There’s also an issue with removing column instances (by removing an instance of each cell subform). In this case, all new instances are removed until what was originally the first instance to be added is removed, leaving only the original, initial instance. What happens is that part of the last instance to be removed remains rendered on the form which doesn’t really look nice because it looks like the instance is still there (even though you can’t click on the cells anymore).
The problem is that the form’s layout isn’t properly updated after columns are added or removed.
Luckily, there’s the xfa.layout object which gives us a solution to this problem when version of Acrobat prior to 8.0 are used to render the form. More specifically, versions 7.0.5+ since table support didn’t appear until then, when Designer 7.1 was released.
Using the xfa.layout object, you can tell Acrobat to re-render the form at your command. This effectively repaints the entire form and gets rid of any artifacts (those being the incorrectly rendered column instances). So, after adding or removing column instances, just place the following command:
Please use this with caution, however, since it may adversely affect the performance of your form (since this statement will essentially cause the entire form to be re-rendered every time a column is added/removed). That’s why I check the version of Acrobat that’s rendering the form in my sample so that I know whether I need to apply the workaround or not.
The other workaround is simply to use Acrobat 8.0 to render the form (save is as an “Acrobat 7.0.5 Dynamic PDF Form” but open it in Acrobat 8.0). Acrobat 8.0 now properly renders all instances as they get added or removed.
Please refer to the Bug List for updated information on the version(s) affected by this bug as well as if and when it was/will be fixed.